1. Field of the Invention
This invention relates to sound systems, and more particularly to circuits for maintaining the stability of a sound system against ringing sometimes caused by acoustic coupling between the input and the output of the sound system.
2. Description of the Prior Art
Sound systems are often used to provide reinforcement for the speaking voice or musical instruments. In this application, there is always some acoustic coupling between the loud speaker and the microphone so that the entire system forms a closed loop. In order for this loop to be stable, that is, not to go into oscillation, it is necessary that at any frequency for which the overall phase shift is an integral multiple of 2.pi. radians, the loop gain must be less than one. Any time this stability criterion is violated, the system will begin to oscillate or "ring" at the frequencies for which the loop gain exceeds one. This ringing will persist until something is done to reduce the loop gain below one. The control of this system is accomplished in actual practice by having an operator present. The human hearing mechanism is such that the operator can distinguish the sounds produced by the system in normal operation from sounds characteristic of ringing or oscillation. Whenever the operator detects ringing in the system, he corrects it by reducing the gain in the electronic portion of the system until the ringing ceases. The operator has restored stability by reducing the loop gain below one. The role played by the operator in maintaining system stability against ringing would not be necessary if the system had the ability to detect its own ringing and upon detection of the ringing, to reduce the loop gain of the system until the ringing ceased. Accordingly, there is a need for an electronic device which can assume the operator's function as described above.